1. Field of the Invention
The present invention relates to electrical heating devices that use positive temperature coefficient thermistors as self-regulating heaters.
2. Description of the Prior Art
As exemplified in U.S. Pat. No. 4,072,848, electrical heating cables have been used commercially for some time to provide heat to pipes and tanks in cold environments.
Heating cables as disclosed in U.S. Pat. No. 4,072,848 based their temperature control on the use of variable resistance heating materials which provide a self-regulating feature. The heating materials are generally formed into chips made of barium titanate or solid solutions of barium and strontium titanate which are made semiconductive by the inclusion of various dopants. These chips are referred to as positive temperature coefficient thermistors and have a relatively low resistance at low temperatures. As the temperature of the thermistor rises, a sharp rise in the resistance occurs at a point termed the "Curie point". The transition from low resistivity to high resistivity occurs at a relatively sharp point as shown in U.S. Pat. No. 4,072,848. As these chips are well known to those skilled in the art, no further discussion of their construction is necessary.
As a voltage is applied to the thermistor, the thermistor generates heat due to resistance effects. This heat is then transferred to the environment, such as the pipe to which the cable is attached. As the temperature of the thermistor and the surrounding environment increases, the thermistor temperature reaches the Curie point, the heat producing capability of the thermistor is reduced and the thermistor cools down. Thus the thermistor temperature settles on or near the Curie point, with the temperature of the surrounding environment being based on the thermal conductivities of the various materials in contact with the thermistor.
Prior art thermistor-based devices were cables and other similar devices which covered only small lateral areas, even though they could be extended for long distances. While the prior art cables could be shaped in serpentine patterns to cover larger lateral areas, this often resulted in uneven temperature distributions over the surface area and was hard to manufacture.
U.S. Pat. No. 4,330,703 shows several examples of prior art cables utilizing heat generating layers of materials and having electrical conductors formed of metal sheets, grid or meshes. The heat generating materials are located over the entire area of the cable, not in discrete and separated areas as is the practice in thermistor-based cables. Additionally, the electrical conductors are thin, utilized only to supply electrical current to the heat generating materials and not utilized to conduct appreciable amounts of heat.